The sexes of mouse-adapted Nippostrongylus brasiliensis exhibit mutual heterosexual pheromonal attraction during in vitro bioassay. The male locomotor response to living females is dosage-dependent while the female response was significant at all tested living male dosages. No time of day influence was found within the 24-hour cycle on male or female production of pheromone or response to the opposite sex's pheromone. Female helminths produced little pheromone and males were essentially unresponsive to female pheromone until after the final larval molt. Female pheromone production and the male response diminished with age. Male Nippostrongylus chemically inhibit each other's response to female pheromone during male-to-male exposure in a female pheromone gradient or during exposure to large male numbers prior to placement in a pheromone gradient. Maintenance of males in pheromone derived from incubation of females also reduced or eradicated the males' responsiveness to a pheromone gradient emanating from females. This reduction was dependent upon pheromone concentration during preexposure or the duration of preexposure. Male helminths are attracted to living females, female incubates or homogenates and also orient to two female chromatographic fractions of different molecular weights. The heavier fraction serves as an aggregation pheromone since it is produced and responded to by both sexes. The lighter weight component attracts only males and is produced by only females. Males also apparently cause a male density-dependent qualitative or quantitative increase in female pheromone attractiveness.